This invention relates to pulse detection in an input signal containing pulses of various modulation and duration, and more particularly to pulse detection and characterization by optical, analog and digital processing.
A pulse detector receives an input signal containing pulses of duration ranging from a few nanoseconds or less to several milliseconds or more and modulation such as chirped, phase shift keying, and frequency shift keying and an additive Gaussian noise. The output signal of a pulse detector is a signal that identifies the time of occurrence and the duration, i.e. the time interval, and the bandwidth and center frequency, i.e. the frequency interval, of each pulse in the input signal.
Prior systems to detect pulses in an input signal rely upon digital technology. The prior digital systems implemented the well known xe2x80x9cmatched detection matrixxe2x80x9d (MDM) algorithm, or variations of that algorithm, for detecting pulses in an electronic input signal. The MDM algorithm effectively searches for concentrations of energy in the time domain and the frequency domain. The portion of the time record where energy is found indicates the time interval where a pulse may exist. The location of energy in the frequency domain represents the frequency interval where the pulse exists.
In order to accurately find the time interval and the frequency interval, many time and frequency intervals must be searched to detect and locate signal energy. The computational load to search these time and frequency intervals can be far greater than a conventional digital computer can provide. Thus, there is a need for a pulse detection system to efficiently implement the MDM algorithm by searching many time and frequency intervals which is faster and requires less weight, volume, power and cost as compared to previous all digital processors.
In accordance with the present invention, there is provided a method and apparatus for implementing the MDM algorithm utilizing a hybrid optical, electrical analog, and digital processor.
The matched detection matrix (MDM) pulse detector of the present invention utilizes a collimated and coherent beam of light to illuminate a Bragg cell which is driven by an input signal containing pulses of various modulation and duration. The Bragg cell in response to the input signal modulates the collimated beam of light.
An optical plate comprising a plurality of elongated openings having selected lengths is positioned to receive the modulated beam of light after passing through an optical lens configuration. The optical lens configuration is positioned in the path of the spatially modulated collimated light beam to form an image of the beam leaving the Bragg cell on the plane of the optical plate. A holographic element is positioned in the path of the imaged beam and near the optical lens to replicate an image of the Bragg cell aperture a plurality of times in the plane of the optical plate.
A second optical lens configuration in the path of the spatially modulated beam of light is positioned down stream from the optical plate. The second optical lens configuration generates the spatial Fourier transform of the light distribution thereby resulting in the intensity of light in the focal plane of the second optical lens configuration to be proportional to the power spectral density of the signal modulating the Bragg cell as modified by the optical plate. A detector array in the focal plane of the second optical lens configuration responds to the light intensity to identify pulses in the input signal including the location of the time and frequency intervals of each pulse.
Further in accordance with the present invention, there is provided a focal plane processor following each detector to implement the matched detection matrix pulse detector that comprises a frequency integrator for selected ones of a plurality of images on an optical plate. The detector and focal plane processor further include a time integrator for selected ones of the plurality of images on the optical plate. In addition, there is included a tuning command selector for selected ones of the plurality of images.
The time integrator of the focal plane processor comprises a gain control responsive to outputs of each detector in an array on the focal plane. A preamplifier responds to the gain control thereby setting the gain of the preamplifier. The gain control corrects for the gain variations from one detector to the next due to manufacturing inconsistencies and noise spectral variations. The output of each detector is sampled at the Nyquist rate or greater, amplified, stored and subsequently read into a storage element of a multiplexer. The stored samples in the multiplexer are read out and the first storage sample is transferred to a subsequent storage element and the next sample is stored in its place. The next storage sample is then read into the multiplexer and the sum of the samples is computed and read out of the multiplexer. This process is repeated and for each repeat the values will be read from the multiplexer.
The frequency integrator in accordance with the present invention comprises a threshold detector receiving outputs from the multiplexer or the time integrator for comparison to a threshold to identify a frequency band corresponding to a specific time integrator. The time, frequency band, and value resulting from the threshold are transmitted to the tuning command selector. The tuning command selector processes the outputs from the frequency integrator starting with the largest and continuing to the smallest.
The largest is most likely to correspond to a valid pulse. This results in an estimate of the band pass (frequency interval) of a pulse and an estimate of pulse duration and arrival time (time interval).
A technical feature of the present invention is an optical, electrical analog and digital processor for pulse detection implementing the MDM algorithm. The implementation in accordance with the present invention enables data to be processed on a per unit time basis more rapidly than previously implemented all digital techniques. Many more time intervals and frequency intervals can be searched to find the pulse resulting in greater time and frequency location accuracy. In addition, more time and frequency intervals result in a more sensitive pulse detector. A further advantage of the present invention is a method of optical, electrical analog and digital processing for pulse detection in an input signal providing efficient weight, volume, power and costs when compared to previous all digital implementation of a pulse detector.